Unevenness due to the light distribution characteristics of a light source may be reduced in an illuminating optical system, such as a liquid crystal projector, even if a light source having an irregular light distribution, such as a metal halide lamp, a xenon lamp, a halogen lamp, or the like, is used. A system called a light integrator is known that uses one or more lenticular lens arrays arranged in the light path in order to make the light more even.
FIGS. 10(A) and 11(A) are sectional views of portions (namely, from a light source to a polarization converter), of prior art illuminating optical systems. FIGS. 10(B) and 11(B) are views of the components shown in FIGS. 10(A) and 11(A), respectively, as seen from the light modulator side of a projection-type display device.
More specifically, the prior art illuminating optical systems of FIGS. 10(A) and 11(A) each includes a light source 910 formed of a lamp 904 and a reflector 901, a first integrator plate 911A (formed of a first lenticular lens array), a second integrator plate 911B (formed of a second lenticular lens array), a polarization converter 911C and a field lens (not shown), listed in the order that light progresses through the system. The first integrator plate 911A is configured by arranging a plurality of lens elements into a two-dimensional array to form a lenticular lens. Similarly, the second integrator plate 911B is configured by arranging a plurality of lens elements into a two-dimensional array to form a lenticular lens. The first integrator plate 911A divides the single beam from the reflector 901, which single beam has a large spatial unevenness of luminosity, into multiple divided light beams, with the number of divided beams being the same as the number of lens elements in the first integrator plate 911A. The spatial variation of the luminous flux in the divided beams is smaller than the spatial variation of the single luminous flux before the division. Each divided beam is then incident onto a respective region of the second integrator plate 911B. The second integrator plate and a field lens operate to direct each divided light beam so that all divided light beams overlap one another at an illuminated area, thereby achieving an even illumination in the illuminated area.
Moreover, each member is arranged within an optical system so that the second integrator plate 91lB and the pupil of a projection lens are at conjugate positions of the optical system and so that the lamp(s) 904 and the first integrator plate 911A are at conjugate positions of the optical system, thus providing a projection-type display device wherein the second integrator plate serves as a secondary light source. If the light modulator is formed using a liquid crystal light modulator, a liquid-crystal projection-t ype display is thus provided.
The prior art polarization converter 911C is arranged, relative to the first integrator plate and the second integrator plate, nearer the second integrator plate 911B. Images of the light source are formed on the second integrator plate, and the polarization converter efficiently converts the unpolarized light from the light source into linearly polarized light having a single polarization direction. Such a polarization converter is disclosed in U.S. Pat. No. 5,986,809 and includes pairs of prisms with each prism supporting a polarizing beam splitter film 971 or a reflection film 972 inclined at an angle relative to an optical axis (FIGS. 10(A) and 11(A)). These films are alternately arranged in a row across the light beam. Thus, there are alternately arranged across the light beam polarizing beam splitter surfaces and reflecting surfaces. Such a structure is herein termed a xe2x80x9ccomb polarization-separation prism arrayxe2x80x9d. Further, half-wave phase retardation plates 963 (hereinafter termed half-wave plates) are arranged on every surface from which the polarized light exits from the comb polarization-separation prism array (i.e., in those light paths of either the transmitted p-polarized component or the reflected s-polarized component so as to rotate the polarization of one of these components 90 degrees, thereby converting the unpolarized light incident on the polarization converter into light exiting the polarization converter that is uniformly polarized.) In FIGS. 10 and 11, half-wave plates 963 are arranged only on those surfaces where the p-polarized light exits the comb polarization-separation prism array. Of course, alternatively, the half-wave plates 963 could be arranged instead on those surfaces where the s-polarized light exits the comb polarization-separation prism array.
Japanese Laid-open Patent Application 8-304739 uses an integrator plate having such a lens array, so that the unevenness of illumination onto a light modulator is reduced. Further, the illumination optical system is compact and provides a bright image. By combining an integrator plate and a polarization converter in such a manner, unpolarized light emitted from a light source may be illuminated onto an illuminating area as light having a single polarization without any significant loss of light. Thus, the light from a light source is efficiently used.
Moreover, a light source, a polarization converter and other members are arranged so as to locate an image of the light source(s) near the polarization-separation film of the polarization converter. Thus, even when the unpolarized light from the light source is split into p-component and s-component light fluxes, the luminous flux is not widened and a compact optical system may be provided.
When the shapes of the light source images formed on the second integrator plate are significantly different (depending on the locations of lens array elements, such as near the optical axis versus at the periphery of the array) light is often wasted. Sections having low illumination efficiency may be formed in a polarization converter having polarizing beam splitter films and reflection films arranged only in one direction.
Additionally, when the shapes of each lens array element of the second integrator plate are different and, particularly, when each lens array element is arranged with a relatively fine pitch in the same direction as the arrangement direction of the polarizing beam splitter films and reflection films, a polarization converter has to have its polarizing beam splitter films and reflection films arranged with a correspondingly fine pitch. This is required in order to provide a high illumination efficiency and for compactness.
An illuminating optical system is disclosed in Japanese Laid-open Patent Application 11-108909 by the present applicant. This illuminating optical system uses a plurality of light sources and has a large spatial variation of luminosity among the light sources. Thus, the shapes of the lens array elements of the second integrator plate differ. However, even with a single light source, the luminous flux has a large spatial variation so that the shapes of the lens array elements of the second integrator often differ.
The present invention provides a polarization converter which has a simple structure that can convert unpolarized light to polarized light having a uniform direction of polarization even when the shapes of light source images formed on a second integrator plate are significantly different from each other, due to the location of lens array elements, and even when the shapes of the lens array elements of the second integrator plate differ. The present invention also provides the above-described polarization converter in combination with a projection-type display device.
The polarization converter of the present invention has at least two regions, with each region including adjacent prisms arranged side-by-side in a row. The prisms support films along the row that are arranged in pairs, each film pair including a polarizing beam splitter film and a reflection film. The row of adjacent prisms in one region is oriented non-parallel to the row of adjacent prisms of another region, and the polarization converter further includes a half-wave plate arranged on an exit surface of every other prism in a given row. Thus, in each region, prisms are arranged as in prior art polarization converters, namely, side-by-side in a row, with adjacent prism surfaces having alternating films applied thereto and arranged in pairs. As in prior art polarization converters, the alternating films arranged in pairs are a polarizing beam splitter film, that reflects only the s-component of incident light, and a reflecting film, that redirects the reflected s-component in the direction of the p-component light transmitted by the polarizing beam splitter film. As in the prior art, half-wave plates are arranged on alternate exit surfaces of the adjacent prism pairs so as to rotate the polarization of the s-component light 90 degrees and thereby convert the light exiting the half-wave plates into p-component light. Alternatively, the half-wave plates can be arranged on different, alternate exit surfaces of the adjacent prism pairs so as to rotate the polarization of the p-component light into s-component light. In each case, just as in the prior art, the polarization converter efficiently converts unpolarized light into light that is linearly polarized in a uniform direction.
The present invention differs from the prior art by having the row direction of one region of the at least two regions be non-parallel to the row direction of another region. Preferably, the row directions differ by an angle of substantially 90 degrees. In addition to the row directions being different among at least two regions, the pitch of the array of prisms in one region will, in general, be different than the pitch of the array of prisms of another region. Further, having the pitch of the adjacent prisms in one region be different from the pitch of adjacent prisms in another region results in the height of the at least two regions of the polarization converter being different, as measured in the direction that light is incident onto the polarization converter.
Also, a projection-type display device of the present invention includes a light source section (wherein a single lamp or a plurality of lamps is arranged); an integrator section having at least one integrator plate wherein a plurality of lens array elements are arranged so as to provide an even illumination; a light modulator which modulates the light emitted from the integrator section in response to predetermined image information, and a projection lens which projects optical images, using the light modulated by the light modulator, onto a screen. The polarization converter mentioned above is arranged near the integrator plate which is closest to the light modulator. Depending on the configuration of the light source and the integrator section, the polarizing beam splitting films and reflection films of the polarization converter may need to have an extremely fine configuration in certain regions but not in other regions. The present invention enables this to be accomplished while taking into consideration cost of manufacture and the precision of assembly needed for a particular design of the polarization converter.